130-Foot Run: PVC/Romex or 3-wire/EMT??

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  #1  
Old 12-25-06, 08:44 PM
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130-Foot Run: PVC/Romex or 3-wire/EMT??

I'm going to run wire from my main panel to my garage. The details can be seen at (13K PDF file):

http://home.bresnan.net/~pipsisiwah/garage_subpanel.pdf

I'm looking for the most economical method and would appreciate any suggestions and comments...

Thanks in advance,

The Pipsisiwah
 
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  #2  
Old 12-25-06, 10:59 PM
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Sorry, that link is no good.
 
  #3  
Old 12-26-06, 01:34 AM
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you needed an extra pipsisiwah...

http://home.bresnan.net/~pipsisiwah/~pipsisiwah/garage_subpanel.pdf

Okay, read your question. You're making a couple small, but significant mistakes. Overall though, you're on the right track.

You cannot use romex/NM for the exterior run. Just forget about it now, even if you have the wiring purchased. This falls under the very bad idea category. You want a product rated for wet use, even though you're considering using PVC. If you go PVC, use THWN. Otherwise, go ahead with the direct burial wiring.

Secondly, you should seriously consider running one more conductor to that subpanel. I see a #10 common, so maybe I can assume you'll separate the neutral from the ground at the subpanel and do it right. It isn't 100% necessary, but if you have any other metal connection btw the garage and the house, you'll need it then. And you can go one size smaller on the wiring, so it isn't too much $.
 
  #4  
Old 12-26-06, 11:11 AM
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The installation that you propose will not work.

You have a choice when feeding a subpanel in a detached structure. You can either run _separate_ equipment ground and neutral conductors ("four wire") or, _if you have no other bonded metallic paths_ between the two structures, you can have a single 'grounded conductor' that serves as both ground and neutral. You have drawn this latter approach.

The problem with this is that creates a 'parallel path' where neutral current will flow along multiple path. The NEC permits this if the parallel path is simply through the soil, and prohibits this if the parallel path is through any other metallic path. But you have a GFCI breaker, and it will sense any current flowing through the parallel path. Very likely the GFCI breaker would trip every time you applied a significant load.

I would not use IMC underground; it will rust away, however this is location specific; you should check with your inspector about suitable underground raceways. I would either use direct bury cable (UF) or I would use THHN/THWN in pvc conduit. Type UF cable is similar to standard NM cable (Romex is a brand name of type NM cable; they also may UF cable), but it is rated for wet and underground locations. Type UF cable can both be buried underground and can be used like normal Romex inside the structure.

You need to consider several things that are not shown in your diagram: 1) location of disconnect in your garage, 2) grounding electrodes in the garage, probably others.

Finally, if you elect to use a four wire feed (I recommend this), then with the conductor sizes that you've selected you would need to run a #10 equipment ground.

-Jon
 
  #5  
Old 12-26-06, 09:34 PM
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First, how did you figure that out (took me awhile!)? You must use Bresnan too.... I've tried and tried to get rid of the extra pipsisiwah at my Bresnan personal web pages but to no avail.

I uploaded a slightly different drawing (contains the green ground). Here is a bit more info:

1. The main box is on the back of the house, underneath the cover over a raised wooden deck. Back of house and the deck face west.

2. I would come out of the main box, underneath the covered deck, through its joists and exit at the end of the deck.

3. The end of the deck is at the corner of the house. From there, I would turn the corner and continue the run north about foot above the ground (a concrete RV parking pad) until it reaches the exterior wall of the garage, then through the garage wall (brick).

4. The garage is attached.

5. The garage is fed by one 15-amp breaker in the main box for lights and door openers.

6. I will pull wire and forget the ROMEX.

7. Unless it is inappropriate or dangerous, I planned on using PVC pipe over IMT to save a few bucks.

8. I may, at times, want to run my 115VAC 15A compressor, a one-lunger that is notorious for current demand when restarting while pressurized. (It actually dropped the line voltage from nominal 110 to 80 V during a pressure-restart) and that is the reason for using #10 wire throughout.

9. If I use IMT and compression fittings, will that suffice for the ground return?

10. I did plan on connecting the ground and common ONLY inside the main box.

11. Finally, I'll use GFI receptacles inside the garage rather than GFI breakers.

I apreciate your help and suggestions. Thanks to you and the others who responded and who have helped me in the past! With your help I can get a nice, clean, safe power source to the garage.


Originally Posted by Fubar411 View Post
you needed an extra pipsisiwah...

http://home.bresnan.net/~pipsisiwah/~pipsisiwah/garage_subpanel.pdf

Okay, read your question. You're making a couple small, but significant mistakes. Overall though, you're on the right track.

You cannot use romex/NM for the exterior run. Just forget about it now, even if you have the wiring purchased. This falls under the very bad idea category. You want a product rated for wet use, even though you're considering using PVC. If you go PVC, use THWN. Otherwise, go ahead with the direct burial wiring.

Secondly, you should seriously consider running one more conductor to that subpanel. I see a #10 common, so maybe I can assume you'll separate the neutral from the ground at the subpanel and do it right. It isn't 100% necessary, but if you have any other metal connection btw the garage and the house, you'll need it then. And you can go one size smaller on the wiring, so it isn't too much $.
 
  #6  
Old 12-26-06, 09:40 PM
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Looks like the installation will be 4 wires!

1. The disconnect in the garage will be just above the receptacle. The run between the disconnect (subpanel?) and where the run enters the garage is 15 feet.

2. Not aware of any grounding electrodes in the garage. If any exist, they are hidden behind the drywall (finished garage).

Originally Posted by winnie View Post
The installation that you propose will not work.

You need to consider several things that are not shown in your diagram: 1) location of disconnect in your garage, 2) grounding electrodes in the garage, probably others.

Finally, if you elect to use a four wire feed (I recommend this), then with the conductor sizes that you've selected you would need to run a #10 equipment ground.

-Jon
 
  #7  
Old 12-26-06, 09:42 PM
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Yes, so I see. I tested this one and it works:

http://home.bresnan.net/~pipsisiwah/~pipsisiwah/garage_subpanel.pdf

I also provided more information to one of the other fellows helping me that may clarify a few things.
 
  #8  
Old 12-27-06, 06:05 AM
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Since this is an _attached_ garage, the rules are somewhat different. I made an assumption rather than asking.

1) In an _attached_ garage, you don't even need the subpanel, and as you have drawn things, you will never make use of the 30A breakers in the main panel. In this case, I would simply place 20A breakers in the main panel, and ditch the subpanel.

2) The ground return path _could_ be the IMC...but I would prefer to use PVC conduit and a #10 green wire as the ground return path. Since people tend to find they need more power in their garage, I would oversize the PVC conduit to provide for future expansion.

3) I recommend using a double pole 20A breaker to supply this circuit, rather than separate single pole breakers. This will give you the option of adding a 15 or 20A 240V receptacle in addition to the 15 and 20A 120V receptacles that you are already adding.

4) If your compressor can be re-configured for 240V operation (check its manual) then do so. This will reduce any voltage drop issues by 75%. You are already planning on a circuit that can supply 240V, you might as well use it.

5) For an _attached_ structure you don't need a separate disconnect, and you don't need separate grounding electrodes, and you can run as many circuits as you wish. For a _detached_ structure, you need a disconnect near the entrance to the structure, you need grounding electrodes, and you are limited to how many circuits/feeders you can supply to the structure.

6) For your conduit run, make sure you know the rules about conduit installation; these include (but you will need to read up on this):
no more than 360 degrees of bend between 'pull points'
expansion fittings on long runs subject to temperature change
sufficient support and mounting
arranged to drain any water they collect
outdoor conduit is always considered a wet location and must use wet rated conductors
conduit must be fully assembled and installed prior to pulling conductors into it

-Jon
 
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